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AR# 37871

XC9500XL Timing - When constraining a signal in a low power MacroCell it fails timing, but it passes when the MC is not low power. Why?

Description

When constraining a signal in a low power MacroCell it fails timing but it passes when the MC is not low power why?

Solution

This is a known issue. The problem is that the tools do not optimize correctly for a low power MacroCell. To work around the issue, the signal should be over constrained for example, the following fails with a slack of -1.5nS: TIMESPEC "TS_ENABLES" = FROM "en" TO "FFS" 21 ns; However, if it is over constrained as follows, it passes with a slack of 100ps: TIMESPEC "TS_ENABLES" = FROM "en" TO "FFS" 12.5 ns;